Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation. Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Solar cells, also known as photovoltaic (PV) devices, are specifically used to generate electrical power. PV devices are used to drive power-consuming loads to provide, for example, lighting, heating, or to operate electronic equipment such as computers or remote monitoring or communications equipment. These power generation applications also often involve the charging of batteries or other energy storage devices so that equipment operation may continue when direct illumination from the sun or other ambient light sources is not available. As used herein the term “resistive load” refers to any power consuming or storing device, equipment or system.
Traditionally, photosensitive optoelectronic devices have been constructed of a number of inorganic semiconductors, e.g. crystalline, polycrystalline and amorphous silicon, gallium arsenide, cadmium telluride and others. Herein the term “semiconductor” denotes materials that can conduct electricity when charge carriers are induced by thermal or electromagnetic excitation. The term “photoconductive” generally relates to the process in which electromagnetic radiant energy is absorbed and thereby converted to excitation energy of electric charge carriers so that the carriers can conduct, i.e., transport, electric charge in a material. The terms “photoconductor” and “photoconductive material” are used herein to refer to semiconductor materials which are chosen for their property of absorbing electromagnetic radiation of selected spectral energies to generate electric charge carriers. Solar cells are characterized by the efficiency with which they can convert incident solar power to useful electric power. Devices utilizing crystalline or amorphous silicon dominate commercial applications and some have achieved efficiencies of 23% or greater. However, efficient crystalline-based devices, especially of large surface area, are difficult and expensive to produce due to the problems inherent in producing large crystals without significant efficiency-degrading defects. On the other hand, high efficiency amorphous silicon devices still suffer from problems with stability. Present commercially available amorphous silicon cells have stabilized efficiencies between 4 and 8%.
Recently, organic materials have attracted considerable interest for use in photovoltaic cells and photodetectors. (C. W. Tang, “Two-layer organic photovoltaic cell,” Appl. Phys. Lett., vol. 48, no. 2, pp. 183–185, January 1986; P. Peumans and S. R. Forrest, “Very-high-efficiency double-heterostructure copper phthalocyanine/C60 photovoltaic cells,” Appl. Phys. Lett., vol. 79, no. 1, pp. 126–128, July 2001; P. Peumans, V. Bulovic, and S. R. Forrest, “Efficient, high-bandwidth organic multilayer photodetectors,” Appl. Phys. Lett., vol. 76, no. 26, pp. 3855–3857, June 2000, each of which are herein incorporated by reference.)
The ability to make large area, ultrathin devices due to a small optical absorption length of ˜500 Å in the visible spectrum and the ability to obtain acceptable photovoltaic conversion efficiencies with economical production costs are among many reasons for interest in these devices. In addition, their compatibility with rugged, conformable, or flexible substrates opens up many applications that cannot be addressed using other conventional detector technologies. A typical known photovoltaic device configuration is the organic bilayer cell. In the bilayer cell, charge separation predominately occurs at the organic hetero-junction. The built-in potential is determined by the HOMO-LUMO energy levels for such a hetero-junctions produce a gap offset between the donor and acceptor layers and produce an electric field around the donor/acceptor interface.
Position sensitive detectors (PSDs) are an important class of photodetectors that use the lateral photoeffect to detect the position of a focused incident light beam. (J. T. Wallmark, “A new semiconductor photocell using lateral photoeffect,” Proc. IRE, vol. 45, no. 4, pp. 474–483, April 1957.) Position sensitive detectors are commonly used in robotic vision, machine tool alignment, and guidance system applications.
PSDs are commonly configured as photodetector arrays fabricated from silicon. A disadvantage of this configuration is the inability to continuously detect a signal without their resolution being limited by detector size. This problem can be overcome by configuring a thin film one dimensional PSD, which has the further advantage of requiring only two outputs, whereas arrays require data output from each detector.
Hydrogenated amorphous silicon (a-Si:H) films have been widely studied for use in large-area PSDs. (S. Arimoto, H. Yamamoto, H. Ohno, and H. Hasegawa, “Hydrogenated amorphous silicon position sensitive detector,” J. Appl. Phys., vol. 57, no. 10, pp. 4778–4782, May 1985; E. Fortunato, G. Lavareda, R. Martins, F. Soares, and L. Fernandes, “Large-area 1 D thin-film position-sensitive detector with high detection resolution,” Sensor Actuat. A: Phys., vol. 51, no. 2–3, pp. 135–142, February 1996; J. Henry and J. Livingstone, “Thin-film amorphous silicon position-sensitive detectors,” Adv. Mater., vol. 13, no. 12–13, pp. 1023–1026, July 2001.)
The a-Si:H films are advantageous over previously used crystalline silicon since they can be made at lower cost and with higher surface area than the crystalline PSDs, however, the size limitation and fabrication cost of a-Si:H films still substantially limits the usefulness of silicon-based PSDs.
A biological substrate, bacteriorhodopsin, has been used to create a motion sensitive detector which can measure the position of a moving light stripe over a film of a protein treated with a high-pH buffer by changes in the photocurrent response. (K. Fukuzawa “Motion-sensitive position sensor using bacteriorhodopsin” Applied Optics vol. 33 no. 31, pp. 7489–7495, November 1994.)
However, this device is limited to having a single biological material that must be buffered with a high pH, and has a low spatial resolution (approximately ±1 mm error) which decreases with increased width of the detector which make this device unusable for many applications.
The advantage of using organic thin film PSDs over those fabricated from photodetector arrays is the ability to continuously detect a signal without their resolution being limited by detector size. Furthermore, a thin film one-dimensional PSD requires only two outputs, whereas arrays require data output from each detector.
Therefore, it would be advantageous to provide an organic hetero-junction PSD (OPSD) with performance equal or superior to that found in many a-Si:H detectors, that can be produced as either a one dimensional PSD or a two dimensional PSD. This PSD would have a high operational bandwidth capable of tracking rapidly scanned optical beams, a resolution substantially similar to or better than the silicon-based detectors, and be produced more economically than existing PSDs.